The invention relates to a process of measuring a distortion angle of weft in textile web, and an apparatus employing said process.
Textile web leaving a loom or a knitting machine has its weft perpendicular to its wrap, but later in the finishing process the weft gets distorted.
Just before drying, the fabric is straighten by a straightening machine controlled by a signal from an apparatus for measuring an angle of weft distortion.
U.S. Pat. No. 3,517,204 describes an apparatus in which an optical system constructed of a single cylindrical lens, slotted aperture stop and a photocell located in front of a light source with the fabric running in between them.
The optical system is reciprocally rotated by an electrodynamic drive system a several degrees about a lateral line perpendicular to the wrap, later called a "central angle", with a rate close to its mechanical resonance frequency. The movement of the optical system is later called "scanning". The light from the light source passes through the fabric and enters the optical system where it is converted to an electric signal by the photocell.
Output signal from the photocell is summed during a time when the optical system is off the central angle with reversed polarity on each side of said center angle.
When the fabric is straight, the output signal is equal to zero because signal from the fabric picked up by the photocell is symmetrically distributed around the central angle; otherwise the output signal is negative or positive. The output signal is not proportional to the angle of distortion, and its absolute value depends upon many factors. Since the range and rate of rotation is constant, determined by the property of the system, there are cases when the output signal is related to a pattern of the fabric rather than to its weft. This happens when the fabric's pattern is within the range of scan. It causes the straightening machine to follow the pattern of the fabric, not its weft.
U.S. Pat. No. 4,786,177 describes an apparatus in which an optical system similar to the one described above is reciprocally rotated about the central angle in steps by a stepper motor controlled by a computer which also processes a signal from the photocell. Position of the stepping motor's shaft directly corresponds to a specific angle at which a sample of the photocell signal is taken. Since the number of samples determined by the number of steps is limited, to improve resolution in this method, the computer finds at what real angle the photocell signal reaches the maximum corresponding to the position of the fabric weft by employing interpolation. The angle of distortion is then converted to a voltage, thus the output signal is proportional to the angle of distortion.
Moreover, in this method, it is possible to change the range and rate of reciprocal rotation of the optical system to cut off unwanted signals coming from the fabric's pattern, which is useful in sensing some fabrics.
The above described method and apparatus has the disadvantage that in hard to read fabrics the computer finds the maximum of the signal at very different angles. Thus output signal is inconsistent, erratic or sometimes wrong.